Quantum cryptography is believed to be a natural candidate to enhance conventional cryptographies because it can provide ultimate security by the laws of quantum theory. Most of research in this field is centered on point-to-point transmission between two users. At present, quantum cryptography has been successfully achieved in a point-to-point link in optical fiber and free space. However, there are limited achievements on quantum key distribution over network to date. There exist more problems for quantum key distribution over network than that over point-to-point transmission. In fact, it has been thought that it is a difficult problem to distribute quantum keys over network.
FIG. 1 shows a conventional configuration of quantum key distribution over a star network, which exploits four phase shifts of weak pulse strings based on BB84 protocol at transmitter and receiver. In this setup, a transmitter (Tx) and receivers (Rx1-Rx3) use a phase modulator to encode and decode the phase shifts, and the transmitter launches a 3-photon pulse with a phase shift randomly chosen from four phases, (e.g. 0, π/2, π and 3π/2) into the fiber. The pulse is then equally split among the 3 receivers. For measurement, each receiver needs synchronization with the sent pulse. In addition, the setup cannot identify which user should receive the signal because all users in the depicted network can simultaneously receive signals from the sender even if she or he is not the intended receiver. That is to say, this system cannot establish a link just between two specific users to implement quantum key distribution.